Cooling rate effect on the phase transitions in a polymer liquid crystal: DSC and real-time MAXS and WAXD experiments

The importance of the cooling rate for the structural transformations in a main-chain poly(hexamethylene-4,4′-bibenzoate) has been presented. Detailed analysis of the phase transitions, main structural parameters and their temperature changes has been performed by differential scanning calorimetry, real-time middle-angle X-ray scattering and wide-angle X-ray diffraction methods. The thermodynamic nature of the initial transformation into a smectic A phase has been discussed. The material in the smectic state is supposed to be organized in smectic domains. The crystallization from the smectic phase depends strongly on the kinetics. The crystallization inside the smectic domains results into different final structures determined by the cooling rate applied. At the highest cooling rates, only one crystalline form has been observed. Different possible modifications have been discussed for the case: either a γ-polymorphic form or still some mesophase of high order, as a frozen metastable state. There is a possibility that the phase might be also identified as a condis crystal. At decreasing cooling rates, a new crystalline form, named α∗, appears together with the first one. Lowering the cooling rate, the volume fraction of the α∗-polymorph gradually increases, at the expenses of the first form. The interesting feature of the new observed α∗-polymorph is that it has some similarities with α- and δ-phases of the same material. Contrary to the previous observations, no γ⇄α∗γ⇄α∗ transformation has been observed neither during the course of single crystallization nor during the subsequent heating. A model describing the gradual transformation of the material during its temperature treatment has been proposed.

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